ngrep - network grep
ngrep <-hNXViwqpevxlDtTRM> <-IO pcap_dump > <
-n num > < -d dev > < -A
num > < -s snaplen > < -S
limitlen > < -W normal|byline|single|none >
< -c cols > < -P char > < -F
file > < match expression > < bpf
ngrep strives to provide most of GNU grep's common features, applying them to
the network layer. ngrep is a pcap-aware tool that will allow you to specify
extended regular expressions to match against data payloads of packets. It
currently recognizes TCP, UDP and ICMP across Ethernet, PPP, SLIP, FDDI and
null interfaces, and understands bpf filter logic in the same fashion as more
common packet sniffing tools, such as tcpdump
(8) and snoop
- Display help/usage information.
- Show sub-protocol number along with single-character
identifier (useful when observing raw or unknown protocols).
- Treat the match expression as a hexadecimal string. See the
explanation of match expression below.
- Display version information.
- Ignore case for the regex expression.
- Match the regex expression as a word.
- Be quiet; don't output any information other than packet
headers and their payloads (if relevant).
- Don't put the interface into promiscuous mode.
- Show empty packets. Normally empty packets are discarded
because they have no payload to search. If specified, empty packets will
be shown, regardless of the specified regex expression.
- Invert the match; only display packets that don't match.
- Dump packet contents as hexadecimal as well as ASCII.
- Make stdout line buffered.
- When reading pcap_dump files, replay them at their recorded
time intervals (mimic realtime).
- Print a timestamp in the form of YYYY/MM/DD HH:MM:SS.UUUUUU
everytime a packet is matched.
- Print a timestamp in the form of +S.UUUUUU, indicating the
delta between packet matches.
- Do not try to drop privileges to the DROPPRIVS_USER.
ngrep makes no effort to validate input from live or offline sources as it
is focused more on performance and handling large amounts of data than
protocol correctness, which is most often a fair assumption to make.
However, sometimes it matters and thus as a rule ngrep will try to be
defensive and drop any root privileges it might have.
There exist scenarios where this behaviour can become an obstacle, so this
option is provided to end-users who want to disable this feature, but must
do so with an understanding of the risks. Packets can be randomly
malformed or even specifically designed to overflow sniffers and take
control of them, and revoking root privileges is currently the only risk
mitigation ngrep employs against such an attack. Use this option and turn
it off at your own risk.
- -c cols
- Explicitly set the console width to ``cols''. Note that
this is the console width, and not the full width of what ngrep prints out
as payloads; depending on the output mode ngrep may print less than
``cols'' bytes per line (indentation).
- -F file
- Read in the bpf filter from the specified filename. This is
a compatibility option for users familiar with tcpdump. Please note that
specifying ``-F'' will override any bpf filter specified on the
- -P char
- Specify an alternate character to signify non-printable
characters when displayed. The default is ``.''.
- -W normal|byline|single|none
- Specify an alternate manner for displaying packets, when
not in hexadecimal mode. The ``byline'' mode honors embedded linefeeds,
wrapping text only when a linefeed is encountered (useful for observing
HTTP transactions, for instance). The ``none'' mode doesn't wrap under any
circumstance (entire payload is displayed on one line). The ``single''
mode is conceptually the same as ``none'', except that everything
including IP and source/destination header information is all on one line.
``normal'' is the default mode and is only included for completeness. This
option is incompatible with ``-x''.
- -s snaplen
- Set the bpf caplen to snaplen (default 65536).
- -S limitlen
- Set the upper limit on the size of packets that ngrep will
look at. Useful for looking at only the first N bytes of packets without
changing the BPF snaplen.
- -I pcap_dump
- Input file pcap_dump into ngrep. Works with any
pcap-compatible dump file format. This option is useful for searching for
a wide range of different patterns over the same packet stream.
- -O pcap_dump
- Output matched packets to a pcap-compatible dump file. This
feature does not interfere with normal output to stdout.
- -n num
- Match only num packets total, then exit.
- -d dev
- By default ngrep will select a default interface to listen
on. Use this option to force ngrep to listen on interface dev.
- -A num
- Dump num packets of trailing context after matching
- -c cols
- Ignore the detected terminal width and force the column
width to the specified size.
- -P char
- Change the non-printable character from the default ``.''
to the character specified.
- -K num
- Kill matching TCP connections (like tcpkill). The numeric
argument controls how many RST segments are sent.
- match expression
- A match expression is either an extended regular
expression, or if the -X option is specified, a string signifying a
hexadecimal value. An extended regular expression follows the rules as
implemented by the GNU regex library. Hexadecimal
expressions can optionally be preceded by `0x'. E.g., `DEADBEEF',
- bpf filter
- Selects a filter that specifies what packets will be
dumped. If no bpf filter is given, all IP packets seen on the
selected interface will be dumped. Otherwise, only packets for which
bpf filter is `true' will be dumped.
The bpf filter
consists of one or more primitives.
usually consist of an id
(name or number) preceded by one or more
qualifiers. There are three different kinds of qualifier:
- qualifiers say what kind of thing the id name or number
refers to. Possible types are host, net and port.
E.g., `host blort', `net 1.2.3', `port 80'. If there is no type qualifier,
host is assumed.
- qualifiers specify a particular transfer direction to
and/or from id. Possible directions are src, dst,
src or dst and src and dst. E.g., `src foo', `dst net
1.2.3', `src or dst port ftp-data'. If there is no dir qualifier, src
or dst is assumed. For `null' link layers (i.e. point to point
protocols such as slip) the inbound and outbound qualifiers
can be used to specify a desired direction.
- qualifiers are restricted to ip-only protocols. Possible
protos are: tcp , udp and icmp. e.g., `udp src foo'
or `tcp port 21'. If there is no proto qualifier, all protocols consistent
with the type are assumed. E.g., `src foo' means `ip and ((tcp or udp) src
foo)', `net bar' means `ip and (net bar)', and `port 53' means `ip and
((tcp or udp) port 53)'.
In addition to the above, there are some special `primitive' keywords that don't
follow the pattern: gateway
and arithmetic expressions. All of these are described below.
More complex filter expressions are built up by using the words and
to combine primitives. E.g., `host blort and not port
ftp and not port ftp-data'. To save typing, identical qualifier lists can be
omitted. E.g., `tcp dst port ftp or ftp-data or domain' is exactly the same as
`tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
Allowable primitives are:
- dst host host
- True if the IP destination field of the packet is
host, which may be either an address or a name.
- src host host
- True if the IP source field of the packet is host.
- host host
- True if either the IP source or destination of the packet
is host. Any of the above host expressions can be prepended with
the keywords, ip, arp, or rarp as in:
ip host host
which is equivalent to:
- ether dst ehost
- True if the ethernet destination address is ehost.
Ehost may be either a name from /etc/ethers or a number (see
ethers(3N) for numeric format).
- ether src ehost
- True if the ethernet source address is ehost.
- ether host ehost
- True if either the ethernet source or destination address
- gateway host
- True if the packet used host as a gateway. I.e., the
ethernet source or destination address was host but neither the IP
source nor the IP destination was host. Host must be a name
and must be found in both /etc/hosts and /etc/ethers. (An equivalent
ether host ehost and not host host
which can be used with either names or numbers for host / ehost.)
- dst net net
- True if the IP destination address of the packet has a
network number of net. Net may be either a name from
/etc/networks or a network number (see networks(4) for details).
- src net net
- True if the IP source address of the packet has a network
number of net.
- net net
- True if either the IP source or destination address of the
packet has a network number of net.
- net net mask mask
- True if the IP address matches net with the specific
netmask. May be qualified with src or dst.
- net net/len
- True if the IP address matches net a netmask
len bits wide. May be qualified with src or dst.
- dst port port
- True if the packet is ip/tcp or ip/udp and has a
destination port value of port. The port can be a number or
a name used in /etc/services (see tcp(4P) and udp(4P)). If a
name is used, both the port number and protocol are checked. If a number
or ambiguous name is used, only the port number is checked (e.g., dst
port 513 will print both tcp/login traffic and udp/who traffic, and
port domain will print both tcp/domain and udp/domain traffic).
- src port port
- True if the packet has a source port value of port.
- port port
- True if either the source or destination port of the packet
is port. Any of the above port expressions can be prepended with
the keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source port is port.
- less length
- True if the packet has a length less than or equal to
length. This is equivalent to:
len <= length.
- greater length
- True if the packet has a length greater than or equal to
length. This is equivalent to:
len >= length.
- ip proto protocol
- True if the packet is an ip packet (see ip(4P)) of
protocol type protocol. Protocol can be a number or one of
the names tcp, udp or icmp. Note that the identifiers
tcp and udp are also keywords and must be escaped via
backslash (\), which is \\ in the C-shell.
- ip broadcast
- True if the packet is an IP broadcast packet. It checks for
both the all-zeroes and all-ones broadcast conventions, and looks up the
local subnet mask.
- ip multicast
- True if the packet is an IP multicast packet.
- Abbreviation for:
ether proto ip
- tcp, udp, icmp
- Abbreviations for:
ip proto p
where p is one of the above protocols.
- expr relop expr
- True if the relation holds, where relop is one of
>, <, >=, <=, =, !=, and expr is an arithmetic
expression composed of integer constants (expressed in standard C syntax),
the normal binary operators [+, -, *, /, &, |], a length operator, and
special packet data accessors. To access data inside the packet, use the
proto [ expr : size ]
Proto is one of ip, tcp, udp or icmp, and indicates
the protocol layer for the index operation. The byte offset, relative to
the indicated protocol layer, is given by expr. Size is
optional and indicates the number of bytes in the field of interest; it
can be either one, two, or four, and defaults to one. The length operator,
indicated by the keyword len, gives the length of the packet.
For example, ` ether & 1 != 0' catches all multicast traffic.
The expression ` ip & 0xf != 5' catches all IP packets with
options. The expression ` ip[6:2] & 0x1fff = 0' catches only
unfragmented datagrams and frag zero of fragmented datagrams. This check
is implicitly applied to the tcp and udp index operations.
For instance, tcp always means the first byte of the TCP
header, and never means the first byte of an intervening
Primitives may be combined using:
- A parenthesized group of primitives and operators
(parentheses are special to the Shell and must be escaped).
- Negation (`!' or `not').
- Concatenation (`&&' or `and').
- Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation have equal
precedence and associate left to right. Note that explicit and
not juxtaposition, are now required for concatenation.
If an identifier is given without a keyword, the most recent keyword is assumed.
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
Expression arguments can be passed to ngrep as either a single argument or as
multiple arguments, whichever is more convenient. Generally, if the expression
contains Shell metacharacters, it is easier to pass it as a single, quoted
argument. Multiple arguments are concatenated with spaces before being parsed.
Errors from ngrep, libpcap,
and the GNU regex library
output to stderr.
Written by Jordan Ritter <firstname.lastname@example.org>.
Please report bugs to the ngrep's Sourceforge Bug Tracker, located at
Non-bug, non-feature-request general feedback should be sent to the author
directly by email.
ALL YOUR BASE ARE BELONG TO US.